Imagers, particularly CMOS imagers, in the past have had problems with scenes that have a high dynamic range such that parts of the scene have highlights therein. In these portions of the image, the pixels of the sensor become saturated such that the digital pixel outputs are all 1's. To solve this problem, sensors have been designed for enhanced dynamic range. (See, for example, the extended dynamic range imagers discussed in European Patent Office Applications EP 1096789 A2 and EP 1096790 A2 and incorporated by reference herein). These sensors have a photo detector and a floating diffusion into which excess charge (that above photodiode saturation) is spilled. These extended range sensors output two values for each pixel, the photodiode value and a spillover value that indicates the rate of charge spillover from the saturated photodiode to the floating diffusion. These two different values are used to create the viewable image of the captured scene; when there is no spillover, the photodiode value is used; and when there is spillover, a new pixel value is created based on the sum of the photodiode value and the floating diffusion value multiplied by the ratio of the photodiode integration time to the floating diffusion integration time. As the ratio of the photodiode integration time to the floating diffusion integration time is increased, the magnitude of the multiplier increases. This multiplication causes a contouring artifact to appear in the resultant image. This is a drawback of the current state of the art.
For example, take the case where a photodiode of a conventional imager has a dynamic range of 8 photographic stops. This can be represented in a linear, binary system, with 8 bits of dynamic range. This corresponds to a system dynamic range of 48 db, for the photodiode. Also assume that the floating diffusion has a charge capacity (when transformed from rate of spillover information to pixel information), that represents an additional 5 stops of photographic range, and then this corresponds to an additional 5 upper significant bits of pixel information, 29 db additional dynamic range. The overall dynamic range of this system would be 13 bits, 13 photographic stops, and 77 db. To capture a picture with this imager, it is necessary to operate the imager with 200 lines of integration time for the photodiode and 25 lines of integration time for the floating diffusion. This means that to convert from floating diffusion rate of spillover data to pixel data the floating diffusion rate of spillover data must be multiplied by 8 (200 Lines Pd/25 Lines Fd) and then added to the photodiode data for that pixel, see equation 6 in application EP 1096789 A2.
A typical Extended Dynamic Range (EDR) CMOS Imager with one output will output serial pixel digital data in a single channel 10. This imager provides two values per pixel as depicted in FIG. 1. The first value (Fd) is the signal representing the rate of charge spillover from the saturated photodiode to the pixel floating diffusion (if the light level is high enough to cause saturation of the photodiode). The second value (Pd) of the same pixel is the value on the pixel photodiode. When the photodiode is not saturated, the image information is the pixel value on the photodiode. The residual floating diffusion information in such a situation is the floating diffusion dark current and any signal resulting from of undesirable partial light sensitivity of the floating diffusion; this signal on the floating diffusion is also present when the photodiode is saturated.
Conventionally, a transform is used to convert from floating diffusion rate of spillover information and photodiode value, to pixel final numerical value (see, for example, equation 6 in application EP 1096789 A2). A new pixel final numerical value is created based on the sum of the photodiode value and the floating diffusion value multiplied by the ratio of the photodiode integration time to the floating diffusion integration time. The multiplication is done to convert from rate of floating diffusion spillover information to actual pixel value. When the photodiode is not bloomed (i.e. not saturated), the floating diffusion does not contain image information, and the pixel data is the photodiode data.
Typically, the pixel values are analog-to-digital (A/D) converted and the transform is implemented in digital logic or a digital signal processor. The multiplication by the ratio of the photodiode integration time to the floating diffusion integration time results in an image that has contouring in the high dynamic range areas of the image. The picture pixel information exists only in the sum of transformed floating diffusion information and the photodiode information
What is needed is a system that will remove the contouring caused by multiplying the floating diffusion information by the ratio of the photodiode integration time to the floating diffusion integration time.